It is known in the art to convert the optical frequency (the wavelength) of a laser beam by means of non-linear interaction in an optical crystal.
A laser beam (pump beam) is focused into the non-linear crystal. If the peak power density of the pump beam is sufficiently high, and several conditions within the crystal are fulfilled (for example, adequate non-linearity, phase matching, optical transparency in the relevant wavelengths, etc.), a beam or beams at different wavelengths will be generated.
Various conversion processes are known. The pump laser beam(s) may be converted into shorter wavelength beam(s) in a second harmonics generation or sum frequency mixing processes. The beam(s) may be converted into longer wavelengths beams in a difference frequency mixing or optical parametric generation processes.
In an optical parametric oscillation process, a non-linear crystal is placed within a cavity, pumped at a single wavelength, thereby generating two new wavelengths.
Many non-linear crystals are known in the art. They differ from one other in their optical transmission range, in their non-linear coefficients, in their damage threshold etc. Phase matching between the interacting beams may be obtained, in bulk uniaxial crystals, by utilizing the birefringence of the crystal. Quasi phase matching may be obtained in several ferroelectric crystals by periodic poling of the non-linear crystal, or in a crystal that is grown in a periodically patterned orientation, with a period that compensates for the phase mismatching.
Both the non-linear interaction process and the phase matching condition are polarization sensitive. Hence, usually the conditions within the crystal for conversion of specific wavelength(s) into a different specific wavelength(s) are fulfilled only for a specific light polarization at a time. For this reason a polarized pump laser is used. The generated beams will also be polarized.
Solid-state lasers comprise a lasing crystal placed within a cavity, and a pump light source that excites the lasing crystal. Current solid-state lasers use diode lasers as a pump light source. Novel solid-state lasers use doped optical fibers as the lasing media. Erbium doped fiber amplifiers (EDFA) are used in the optical communication field for generating and amplifying light beams. Such fiber lasers are superior to bulk crystal lasers in their efficiency, rigidity, and plasticity.
Fiber laser systems may be set up to generate narrow line widths that are required for pumping a frequency converter. However, usually they generate non-polarized light. Fiber lasers are also limited in their peak power because of the damage threshold of the fiber.
Fiber laser pumped optical frequency conversion has been tried. Several methods have been utilized to get the polarized pump beam that is required for frequency conversion. It is possible to separate the two polarizations and convert each polarization by itself in a separate converter. Polarizations maintaining fibers or fiber bending configurations have been used to preferably enhance one polarization at the expense of the other. In all those methods, the peak power is less than that obtained from a non-polarized lasing configuration.